1. Field of the Invention
The present invention relates to multilayer piezoelectric ceramic materials. More particularly, the present invention relates to piezoelectric ceramics having a base metal, preferably copper, electrode. Still more particularly, piezoelectric ceramic compositions having a base metal as the electrode are cofired without oxidation of the base metal. Compositions and methods of manufacture of multilayer piezoelectric materials with base metal electrodes, preferably copper, are provided.
2. Description of the Prior Art
By way of background, multilayered piezoelectric ceramic materials have a wide variety of industrial applications. Such applications include actuators, ultrasonic transducers, voltage transformers, capacitors, and ultrasonic motors.
For these multilayered ceramics, it is well known to use noble metals such as Pd or Pt and various Ag/Pd alloys as materials for the inner electrodes. Using suitable processing conditions, the noble metals can be cofired up to 1200 degrees Celsius or even higher with the ceramics during the manufacturing process without losing their electrical properties. This is mainly due to their high oxidation resistance and high melting points. However, a great disadvantage is the high cost of noble metals, especially Pt and Pd. Ag—Pd alloys have problems with silver diffusion and electromigration under processing and high field, and high humidity operating conditions. Therefore, there is a strong need for finding low-cost solutions in the fabrication of multilayer structures, specifically actuators, especially for large scale markets, e.g. for the automotive industry. Recently, there have been successful attempts to cofire BaTiO3 (BT) electronic ceramics with Ni powder as a low-cost base metal electrode material for capacitor devices, but here the sintering parameters required are severe, with relatively high, (firing temps>1200° C. and using a reducing multilayer atmosphere, less than PO2˜10−10 atms). For piezoelectric ceramics, such as ceramics based on Pb(Zr,Ti)O3(PZT) or Pb(Mg1/3Nb2/3)O3 (PMN) with very high sintering temperatures, pure base metal powders such as Ni are not suitable. In this case, copper is the base metal electrode of choice, but to this point, there have been no public reports of successful procedures for copper cofiring with piezoelectric ceramics.
There are significant problems related to the cofiring of base metals, i.e., copper, with piezoelectric materials such as Pb(Zr,Ti)O3 (PZT). Such problems include removal of organic binder material without residual carbon; low firing temperatures (less than melting point of Cu=1033° C.) i.e. flux additions; firing without oxidation of base metals, for example, firing with copper as an electrode without the formation of copper oxide; firing without reduction of PZT or other ceramic material.
The present invention is directed toward finding both suitable materials and processing techniques, which allow cofiring piezoelectric ceramics, such as materials based on PZT, PMN, Bismuth based piezoelectrics, or leadfree ceramics based on BaTiO3, with base metal electrodes such as Cu or Ni (or their respective alloys) in order to get high quality piezoelectric multilayer devices.
In light of current technology, there still remains a long-felt need for materials and methodology suitable for the cofiring of piezoelectric ceramic materials with base metals.
Accordingly, it is an object of the present invention to provide compositions of piezoelectric ceramics suitable for cofiring with base metals.
It is another object of the present invention to provide piezoelectric ceramic materials that can be co-fired with copper or other base metal without oxidation of the base metal and/or reduction of the ceramic.
Another object of the present invention is to provide a methods for the cofiring of piezoelectric ceramic materials with base metal electrode materials.
These and other objects and advantages of the present invention and equivalents therof, are achieved by the methods and compositions of the present invention described herein and manifest in the appended claims.